java - 理解 Java 中对象比较的 == 运算符

Question

我知道 == 运算符会检查相等的引用(地址)，但是在比较 Thread 时，我不知道编译器如何抛出以下错误和 String目的。java: incomparable types: java.lang.Thread and java.lang.String这是我的代码:

public static void main(String[] args) {
    Thread t = new Thread();
    Object o = new Object();
    String s = new String("");

    System.out.println(t == o);//no issues here

    System.out.println(t==s);// but this throws above error
  }

为什么允许比较 Thread和 Object但不是 Thread和 String ?

Answer 1

指定比较不能在它们之间转换的引用类型必须导致编译错误。见 JLS chapter 15.21.3 :

15.21.3. Reference Equality Operators == and !=

[...]

It is a compile-time error if it is impossible to convert the type of either operand to the type of the other by a casting conversion (§5.5). The run-time values of the two operands would necessarily be unequal (ignoring the case where both values are null).